1. Field of the Invention
The present invention relates generally to optical pickup devices, and more particularly to an optical pickup device, which achieves miniaturization and slimness of an optical pickup device by using a two-wavelength light source module and a hologram, and reduces the cost of the optical pickup device by continuing to use a conventional photo detector.
2. Description of the Prior Art
Generally, an optical pickup device reproduces signals recorded in various optical discs or records signals in optical discs. Recently, compact disc (CD)/digital versatile disc (DVD)-compatible pickups have become popular. FIG. 1 shows the construction of a conventional optical pickup device using a two-wavelength light source module.
Referring to FIG. 1, the conventional optical pickup device comprises a two-wavelength light source module 1, a diffraction grating 2, a beam splitter 3, an object lens 4, a sensor lens 5 and a photo detector 6. The two-wavelength light source module 1 generates two beams with wavelengths of 650 nm for a DVD and 780 nm for a CD, respectively. The diffraction grating 2 divides each beam emitted from the two-wavelength light source module 1 into at least three beams such as 0th, +1th, and −1th diffraction order beams. The beam splitter 3 causes inputted beams to be reflected toward an optical disc D. The object lens 4 condenses beams onto a track of the optical disc. The sensor lens 5 concentrates passing beams when beams reflected from the optical disc D pass through the beam splitter 3. The photo detector 6 detects the beams concentrated by the sensor lens 5 and converts the beams into electrical signals.
In the optical pickup device with the above construction, each beam emitted from the two-wavelength light source module 1 is condensed onto a single point on the optical disc D through the diffraction grating 2, the beam splitter 3 and the object lens 4, while each beam reflected from the optical disc D is detected by the photo detector 6 through the object lens 4, the beam splitter 3 and the sensor lens 5.
However, in such a conventional optical pickup device, the positions of beams arriving at the photo detector 6 are spaced apart from each other in proportion to oscillation intervals of beams emitted from the two-wavelength light source module 1. Therefore, a new photo detector having patterns, which are spaced apart from each other in proportion to the oscillation intervals, must be developed and used, instead of a conventional photo detector now being widely used. Moreover, the cost of the optical pickup device is high due to developing costs of the new photo detector.
Further, the conventional optical pickup device is problematic in that it requires many installation processes due to a great number of optical parts, it is difficult to achieve slimness and miniaturization of the optical pickup device, and the manufacturing cost is increased due to its complicated construction.
Therefore, there is recently proposed an optical pickup device using a hologram so as to simplify the construction of the conventional optical pickup device and reduce the number of optical parts constituting the optical pickup device.
FIG. 2 shows a conventional optical pickup device using a hologram. Referring to FIG. 2, the optical pickup device using a hologram comprises a two-wavelength light source module 10 to emit beams, a diffraction grating 12 to divide each beam into three beams, a hologram optical element 14 to receive three beams reflected from an optical disc D and diffract the beams, and a photo detector 16 to receive beams which are diffracted and condensed while passing through the hologram optical element 14. Both the two-wavelength light source module 10 and the photo detector 16 are fixed to a single common board by means of die boding. The diffraction grating 12, the hologram optical element 14, the two-wavelength light source module 10 and the photo detector 16 mounted on the single common board are integrated into a single package. Naturally, an object lens 4 to condense beams onto a single point on the optical disc D is disposed between the hologram optical element 14 and the optical disc D.
In the optical pickup device constructed as described above, each beam emitted from the two-wavelength light source module 10 is divided into three beams by the diffraction grating 12. The three divided beams are condensed by the object lens 4 to be irradiated onto the surface of the optical disc D. The beams irradiated onto the surface of the optical disc D are reflected, and the reflected beams are diffracted by the hologram optical element 14, and are then detected by the photo detector 16.
As described above, in the optical pickup device using a hologram, the beams diffracted by the hologram optical element are detected by the photo detector, thus eliminating both a beam splitter and a sensor lens, and consequently reducing the number of optical parts. Further, the two-wavelength light source module, the photo detector, the diffraction grating and the hologram optical element are integrated into a single package, thereby simplifying the construction of the optical pickup device and reducing the manufacturing costs thereof.
However, in the conventional, hologram optical pickup device, it is necessary to arrange the photo detector in an exact position because tolerance between the optical source module and the photo detector greatly affects the performance of the photo detector which detects beams emitted from the light source module. In this case, the conventional hologram optical pickup device is disadvantageous in that the operation of precisely arranging both the light source module and the photo detector is complicated, and it requires expensive equipment of great precision. Further, the conventional hologram optical pickup device is disadvantageous in that it is impossible to adjust the position of the photo detector after the photo detector is once fixed.
Meanwhile, in the hologram optical pickup device, the hologram must be divided into two pieces so as to diffract two beams of different wavelengths to be condensed onto a single point of the photo detector. In this case, the hologram optical pickup device is problematic in that when the hologram is divided into several pieces in order to obtain a required signal, dispersion noise of beams arriving at the hologram is created, thus decreasing optical efficiency.